1. Field of the Invention
The present invention relates to an optical communication module used for optical communication, etc. using optical fibers, and more specifically, to an optical communication module with a construction in which the light of a waveband other than a required waveband is not allowed to be incident on the light receiving element and the light of unwanted waveband is not returned to the optical transmission line.
2. Description of the Prior Art
The conventional bi-directional optical communication module comprises, for example, as shown in FIG. 6, a light emitting element 21 such as semiconductor laser, etc. for generating transmission signal light, a light receiving element 22 such as a photodiode, photo-transistor, etc. for receiving the receiving signal light via a half mirror 23, a condenser lens 24 for connecting transmission signal light reflected by a half mirror 23 to a optical transmission line 25 such as optical fibers, a light transmission line 25 for transmitting converged light, and a wave separator 26 for reflecting the unwanted waveband light. This light emitting element 21, light receiving element 22, and half mirror 23 are covered with an unillustrated package and a condenser lens 24 serves as an opening window to connect with the light transmission line 25. Under this configuration, the transmission signal light from the light emitting element 21 reflects against the half mirror 23 and is incident on the light transmission line 25 and sent to the counterpart. When the signal sent from the counterpart is received, the receiving signal light penetrating the half mirror 23 from the light transmission line 25 is received by converting it into an electrical signal by the light receiving element 22 and optical communication takes place. In such event, transmission and reception are switched over alternately by time division, and interference between the two does not occur.
For the regular optical communication, the 1.3 xcexcm band light is used but in the light transmission line 25, the 1.55 xcexcm band light for broadcasting or equipment check are frequently mixed at the same time. In such event, in the regular optical communication, the 1.55 xcexcm band light is not wanted and work as an interfering light. Consequently, as shown in FIG. 4, in general, a wave separator 26 comprising an interference filter or multilayer film is inserted before a module to remove the 1.55 xcexcm band light.
As described, when a wave separator for removing the unwanted waveband light is used, the number of parts increases and cost increases as well as the module size increases.
The coating film may be provided on the surface of the half mirror 23 to form the filter, and the 1.55 xcexcm band light may be reflected, but there are problems in that when the light is reflected with the half mirror, the unwanted 1.55 xcexcm band light repeats irregular reflections in a package and returns to the light transmission line 25 and gives noise to the case in which the 1.55 xcexcm band light is being used, or enters the light receiving element 22 and causes noise to enter the receiving signal.
It is an object of this invention to provide an optical communication module which can receive light of a required waveband only without using a special component such as a wave separator and at the same time which has an inexpensive structure to prevent the light of an unwanted waveband from returning to the light transmission line.
The optical communication module according to this invention comprises a transparent member for allowing the light from the light transmission line to transmit, a light receiving element for receiving light passing the transparent member, and a package for covering the light receiving element and using the transparent member as an opening window, wherein on one surface of the transparent member, a wavelength selective film for allowing the light of a required waveband to transmit and reflecting the light of an unwanted waveband is coated.
Now, the light transmission line referred to herein means a medium which can transmit the light such as optical fiber core. And the transparent member means a medium for allowing the light from the light transmission line, such as a condenser lens (rod lens), window glass, etc. to transmit.
With this configuration, the light of the waveband not required for optical communication such as the 1.55 xcexcm band is reflected at one surface of a transparent member near the outer surface side of the package, and has less chance to reflect in the package to reach the light receiving element, and enables the reception of a required waveband with less noise. And if the wavelength selective film is not formed at the focal point of the receiving signal light beam, the reflected light neither returns to the light transmission line as it is nor allows the light of the unwanted waveband to return to the light transmission line.
Specifically, the transparent member formed with the material having the practically same refraction factor as that of the light transmission line is installed in such a manner to come in contact with the light transmission line, and the wavelength selective film is coated on the surface of the transparent member opposite to the light transmission line; or the transparent member is located with a specified clearance provided with the light transmission line, and on the surface of the transparent member on the light transmission line side, the wavelength selective film is coated.
Now, the practically same refraction factor referred to herein is that the refraction factor is close to such a level that the reflection causing problems does not occur on the interface.
When the transparent member is brought in contact with the light transmission line, it is preferable that the transparent member comprises a rod lens and the wavelength selective film formed surface of the rod lens is formed in a curvature so that the light reflected by the wavelength selective film returns into the rod lens, because the light of the unwanted waveband can be reflected outside the package such as the outer circumferential side from the optical fiber core without allowing the reflection light to return to the light transmission line.
It is preferable that the transparent member is a condenser lens because light can be condensed to the light receiving element.
It is preferable that the surface to which the wavelength selective film is formed is a curvature, because the reflected light is difficult to return to the light transmission line.
It is preferable that a light absorbing material is installed in the package so that the light reflected by the wavelength selective film does not scatter in the package, because even when the light of the unwanted waveband is reflected in the package, it does not make multiple reflection and is difficult to reach the light receiving element.
It is preferable that the wavelength selective film is installed on the outer surface side of the package to prevent the light reflected by the wavelength selective film from entering the package, because the reflected light is difficult to enter the package.
If the second transparent member equipped with the wavelength selective film is installed between the transparent member and the light receiving element, it is possible to further eliminate the light of the unwanted waveband.
An optical transmission module that can carry out bi-directional communication of transmission and reception can be obtained by further having a light emitting element within the package.
A specific bi-directional optical transmission module according to this invention comprises a light emitting element for generating transmission signal light, a condenser lens for connecting the transmission signal light from the light emitting element to the light transmission line, a light receiving element for receiving the receiving signal light from the light transmission line, and a package which covers the light emitting element and the light receiving element and uses the condenser lens as an opening window, wherein to the transparent member installed to or in the vicinity of the convex surface of the condenser lens, a wavelength selective film which allows the light of a specified waveband to transmit and reflects the light of the unwanted waveband is provided.
Now, the vicinity of the convex surface of the condenser lens referred to herein means a distance which is less than one half the distance from the condenser lens to the light emitting element or the light receiving element.
Even with this configuration, the light of the waveband not required for optical communication such as the 1.55 xcexcm band reflects on or in the vicinity of the condenser lens and reflects to the light transmission line side. However, since the light does not reflect upon the focal point of the condenser lens, the beam reflecting direction is changed at the convex surface of the condenser lens and does not return to the center portion of the light transmission line. Consequently, the light reflects to the light transmission line side and does not enter the light transmission line, and repeats irregular reflections with the surrounding of the light transmission line and does not enter the light transmission line or light receiving element.